The human c-fes locus encodes a 93 kDa cytoplasmic protein-tyrosine kinase (Fes) that is expressed predominantly in hematopoietic cells of the granulocytic and monocytic lineages. Transfection of the CML-derived myeloid leukemia cell line K-562 with the c-fes gene results in growth suppression and terminal differentiation. These findings argue strongly that Fes plays a major role in myeloid growth regulation and could be targeted for reversal of the differentiation block associated with myeloid leukemia. Structurally, Fes consists of a unique N-terminal region, a Src homology 2 (SH2) domain, and a C-terminal kinase domain. During the previous grant period, the principal investigator and his colleagues found that all three domains contribute to the regulation of Fes kinase and biological activities. In particular, the coiled-coil homology domains found in the Fes N-terminal region play an essential role in kinase regulation in vivo. During the next grant period, Dr. Smithgall will continue to address the role of the coiled-coil domains as regulators of Fes kinase and biological activities. In addition, the PI will investigate mechanisms that induce activation of Fes. In particular, he will focus on cytokine receptors, receptor-associated Jak and Src family kinases, and the oncogenic tyrosine kinase Bcr-Abl as upstream regulators of c-Fes. These broad goals will be addressed with the following Specific Aims: (1) Test the hypothesis that intramolecular interaction of the Fes coiled-coil domains is sufficient for negative regulation of Fes tyrosine kinase activity. (2) Test the hypothesis that interaction of Fes with the GM-CSF receptor beta-chain is sufficient for Fes activation. (3) Test the hypothesis that phosphorylation of Fes by Jak and Src kinases affects Fes tyrosine kinase activity and signaling properties. (4) Test the hypothesis that Fes-induced differentiation of CML cells involves the interaction of Fes with Bcr-Abl. These studies will help to elucidate the molecular mechanisms that regulate Fes tyrosine kinase activity and to identify the signal transduction pathways utilized by Fes to transmit signals for differentiation.